Precise control of plasma processing of semiconductor wafers requires that the wafer temperature be carefully regulated or held very close to a desired temperature. Drift of wafer temperature causes various characteristics of the plasma process to change, so that the process cannot be accurately controlled. For example, in a plasma etch process, the etch rate may increase if the wafer temperature drifts toward a higher temperature. Typically, plasma processing will increase the wafer temperature. In order to maintain the wafer temperature at a desired level, the wafer is cooled during plasma processing by pumping a coolant such as Helium gas through the wafer support through coolant passages which permit the coolant to conduct heat away from the wafer backside. The cooling rate is proportional to the pressure at which the coolant is supplied to the coolant passages in the wafer support, so that the wafer temperature is directly affected by the coolant pressure. Thus, it is conventional to set the coolant pressure to a constant value corresponding to a desired wafer temperature during processing.
Heretofore, it has been assumed that with the foregoing cooling technique, wafer temperature is generally constant during plasma processing of the wafer. This is an important assumption, particularly in the case of nitride processes, because plasma etch processes for etching nitride layers are very sensitive to changes in temperature. Wafer temperature has typically been measured using conventional (commercially available) xe2x80x9ctemperature dotsxe2x80x9d which can be stuck to the surface of a test wafer. At the conclusion of plasma processing of the test wafer, the color of the temperature dot indicates the highest temperature reached during the process. Assuming the wafer temperature is constant over the duration of the plasma process, this temperature is generally taken to be the process temperature of the wafer.
However, it has not been practical to test the assumption of constant wafer temperature during the process. For example, at plasma ignition, it is assumed the wafer temperature climbs very quickly (e.g., within a matter of seconds) from room temperature to the steady-state plasma processing temperature. This assumption could not be verified because direct measurement of wafer temperature during processingxe2x80x94and particularly during processing of production wafersxe2x80x94has not been practical. The only temperature that can be continuously monitored during processing is the wafer support or xe2x80x9cpuckxe2x80x9d temperature, which is at a temperature significantly lower than that of the wafer during processing.
As described below, one aspect of the present invention provides a highly accurate probe with which the temperature of a test wafer having a special dye in a predetermined spot can be continuously monitored during plasma processing. With this probe, it has been discovered that the wafer temperature is not constant during processing, because, among other things, the wafer takes a surprisingly long time (over one minute) to climb from room temperature to steady state process temperature following plasma ignition. Such variations in wafer temperature are detrimental because they tend to reduce the precision with which the process parameters (e.g., etch rate) may be controlled. It was also discovered that, in a plasma reactor employing an electrostatic chuck, the wafer temperature climbs very high near the end of plasma processing. This is because coolant pressure is removed before turning off RF power to the electrostatic chuck holding the wafer on the wafer support. Otherwise, the wafer would be blown off the wafer support by the coolant pressure on the wafer backside as soon as RF power is removed from the electrostatic chuck.
Thus, it is a discovery of the invention that there is a need to sense deviations in wafer temperature from a desired temperature and to somehow correct such deviations. While this is certainly possible in the case of a test wafer whose temperature throughout processing is continuously monitored using the probe of the invention referred to above, it does not seem possible in the case of a production wafer which should not be contaminated with the dye required for the probe to measure wafer temperature.
Thus, there is a need for a way of deducing in real time deviations of the temperature of a production wafer from a desired temperature during processing, without being able to directly measure the wafer temperature. Further, there is a need for a way of changing the system in response to such deviations to minimize or avoid them.
The invention solves the problem of continuously monitoring wafer temperature during processing using an optical or fluoro-optical temperature sensor including an optical fiber having an end next to and facing the backside of the wafer. This optical fiber is accommodated without disturbing plasma processing by providing in one of the wafer lift pins an axial void through which the optical fiber passes. The end of the fiber facing the wafer backside is coincident with the end of the hollow lift pin. The other end is coupled via an xe2x80x9cexternalxe2x80x9d optical fiber to temperature probe electronics external of the reactor chamber. In a preferred embodiment, the hollow lift pin is supported with the other lift pins on a lift spider and a flexible bellows assembly. The optical fiber inside the hollow lift pin and the external optical fiber are preferably coupled together by a flexure near the bottom of the bellows. Preferably, a cavity smaller than the diameter of the optical fiber is drilled in the wafer backside in registration with the optical fiber inside the lift pin, and a suitable dye is deposited in the cavity to facilitate temperature sensing by the sensor.
The invention also solves the problem of determining wafer temperature deviations in production wafers in which there is no cavity nor dye in the wafer backside enabling temperature measurement by the optical probe. The invention solves this problem by first, using direct wafer temperature measurements with a test wafer, establishing a data base of wafer temperature behavior as a function of coolant pressure and establishing a data base of wafer temperature behavior as a function of wafer support or xe2x80x9cpuckxe2x80x9d temperature. These data bases are then employed during processing of a production wafer to control coolant pressure in such a manner as to minimize wafer temperature deviation from the desired temperature.
In a preferred embodiment, such control of the coolant pressure is accomplished by first measuring puck temperature and using the data base of wafer temperature as a function of puck temperature to deduce the wafer temperature from the measured puck temperature. Then, the wafer temperature thus deduced is compared with a desired temperature to calculate an error. This error is used with the data base of wafer temperature as a function of coolant pressure to determine a change in the coolant pressure which will tend to correct the error in the manner of a feedback control system.
In another embodiment, the corrections to the coolant pressure are established in a trial-and-error method. Thus, for example, the delay in wafer temperature rise immediately after plasma ignition is corrected by a corresponding delay in applying or increasing the coolant pressure immediately after plasma ignition, the coolant pressure being gradually increased in accordance with a schedule (of coolant pressure as a function of time after plasma ignition) that permits the wafer temperature to increase very quickly to the desired temperature and remain there. The trial-and-error method of establishing the schedule of coolant pressure is carried out with a test wafer using the optical wafer temperature probe of the invention. The coolant schedule is modified over successive attempts until a fairly constant wafer temperature from plasma ignition onward is achieved. Then, the schedule of coolant pressure is applied to production wafers.
In a yet further embodiment, the wafer temperature behavior observed in test wafers with the optical probe of the invention is parameterized in an equation as a function of coolant pressure and puck temperature. This equation is then employed to accurately calculate coolant pressure corrections based upon continuously measured puck temperature.
The problem of wafer temperature increase just prior to wafer de-chuck from the electrostatic chuck (i.e., when the coolant pressure is turned off) is solved by reducing the RF power on the electrostatic chuck to a level at which heat transfer to the wafer is reduced but which is still sufficient to electrostatically retain the wafer on the chuck.